The present invention relates to postal meters and more particularly to an electronic postal meter having an improved, noise-rejection input/output channel.
Electronic postal meters have been developed utilizing microprocessors as a part of the meter control unit. Data and instructions may be entered into the control unit for such meters through keyboard devices. The results of calculations, requests for more information and error messages may be presented to an operator on an output printer or on a CRT display unit. Units such as the keyboard, the printer and the CRT display, generally described as input/output devices may be located at some distance from the meter control unit and the meter mechanism controlled by that unit, requiring some form of communications channel between the input/output devices and the meter control unit. Heretofore, the communications channel consisted of direct electrical connections in the form of electrical cables or leads between the computer control and the input/output devices.
Postal meters are generally located in the vicinity of other electrical machines which, during operation, may produce extraneous electric fields. Such extraneous electric fields may induce noise voltages in nearby electrical apparatus and particularly in cables or leads. Where the apparatus operates with low signal voltages, as is the case for a microprocessor, induced noise voltages may cause the apparatus to misinterpret and erroneously act upon incoming information.
Moreover, postal meters are most likely to be found in business offices. Since many business offices are carpeted, users of postal meters may build up a static electric charge simply in walking to the meter. When the user touches the keyboard or other input unit, the static electrical discharge may temporarily cause a controlling microprocessor to malfunction or to misinterpret incoming data.
Shielded cables have been used to shield electrical connectors from extraneous electric fields. However, such shielded cables do not solve another problem; i.e., the effect of an electrical malfunction or voltage surge generated in an input/output device such as a keyboard. When a malfunction occurs or a voltage surge takes place in such a device, the voltage may be transmitted directly to the microprocessor control. Voltage surges may disrupt microprocessor operation or even destroy microprocessor circuitry.
Moreover, it is possible for a remote postal meter to be disconnected from one input/output device and reconnected to another. Where the meter and the control unit are directly connected, a faulty reconnection may cause damaging voltages to be applied to the meter.